Provide sandbox of private access warnings

[trombonehero]

Currently, SOAAP's private_access warnings tell us what sandbox the data being accessed was supposed to belong to (with the sandbox_private) field. I would also like to know what sandbox the access actually occurred in.

[trombonehero]

It would be great if the representation for this could be consistent with what we do in a vulnerability_warning, i.e.:

{
  "function": "_ZNK6SkMask16computeImageSizeEv",
  "sandbox": "capsicum",     // or, as would often be the case, "sandbox": null
  "location": {
    "file": "../../third_party/skia/src/core/SkMask.cpp",
    "line": 23
  },
  /* ... */
  "trace_ref": "!trace0"
}